Hippocampus Flashcards
What creature does the hippocampus resemble?
Sea horse
Where is the dentate gyrus?
It appears to be at the head of the seahorse.
It’s a tooth-like bump
Where is the entorhinal cortex?
At the tail of the seahorse
What is the entorhinal cortex’s role?
Input and output.
Where are CA1 and CA3 and what are they called?
The body of the seahorse and can be called Ammon’s horn
What is the basic circuit of the hippocampus (input)?
Input from the entorhinal cortex goes by the layer II and layer II perforant path.
Layer three goes to CA1.
Layer II goes to DG and CA3. Once in the DG or CA3 it continues on to CA3 and then CA1.
CA3 has a feeback loop.
What connects information traveling from the DG to CA3
Mossy fibers. It’s one of the largest and most powerful synapses in the brain.
DG
Dental Gyrate
What is the feedback loop for CA3 called?
Recurrent collaterals
What is the path from CA3 to CA1
Schaffer collaterals
What is the Hippocampal System of that includes output?
It’s the same as input then
CA3 can feedback to the Hilus which can feed to the DG and vice versus. DG feeds to the EC which feeds back to the subiculum.
CA1 also feeds directly to the subiculum which projects back to layer V.
EC also feeds to other cortical and subcortical areas.
What is in the hippocampus proper?
DG/Hilus, CA3, CA1
EC
entorhinal cortex
sub
subiculum
What is the first and second input/output pathway for the hippocampus?
1st: Entorhinal cortex
2nd: Alveus/Fimbria/Fornix
What does the Alveus/Fimbria/Fornix communicate with?
septal nuclei and mammillary bodies
Why does the Alveus/Fimbria/Fornix have three names?
It changes names along the path.
What is known about oxygen and the hippocampus?
It’s the most sensitive area in the brain to oxygen. This is seen in that almost drowning can also lead to anterograde amnesia.
What is the cognitive map in the hippocampus?
It’s not like maps in the rest of brain.
Place cells in the hippocampus fire for where a person is located, but if the person moves, they cells next door don’t necessarily fire.
What was discovered about place cells in the hippocampus?
They are tuned to location.
They do not change firing if the rat turns around. It fires for place independent of where the rat is looking.
What is the hippocampal controversy?
Is it a spatial or memory processor.
Why evidence is there that the hippocampus is a spatial processor?
Lesioned rats were impaired on tasks requiring them to recognize cue configurations
What is David Marr’s theory of how the hippocampus encodes memory?
It associates inputs from different sensory modalities into a unified whole.
How many items does David Marr’s theory of the hippocampus say it can store?
~10,000 items via rapid storing
What does David Marr’s theory say about retrieval in the hippocampus?
associative retrieval can happen from a partial memory
What does David Marr’s theory say about short-term memory and the hippocampus?
The hippocampus is for short-term storage and memories are moved to the cortex during sleep by being replayed.
How does Marr’s say associate recall works?
Circuitry does pattern completion
How are memories separated according to Marr so that they can be distinguished from one another?
Conjunctive subsampling plus thresholding gives orthogonalization for better pattern separation.
What is a problem with Marr’s theory?
If he’s right, we have one memory per second and that is 100,000 snapshots per day. It would require a lot of storage.
How can spatial working memory be tested?
Radial arm maze
How do hippocampal lesioned rats do on the radial arm maze?
They are severely impaired.
What study shows that hippocampus is used for sequence dependent memory (non-spatial tasks)
A rat has to select cups with odors in a certain order to get the reward or a rat has to discriminate between odors. Lesioned rats are bad at the first and normal on the second.
In fact, lesioned rats are even normal when they have to learn the set and then select the cup with an odor that was not in the set.
What is Hebbian learning?
Hebbian learning is one of the oldest learning algorithms, and is based in large part on the dynamics of biological systems. A synapse between two neurons is strengthened when the neurons on either side of the synapse (input and output) have highly correlated outputs. (Fires together wires together)
What does McClelland say about how the hippocampus is able to story episodic memories?
A one-shot Hebbian learning process
What does McClelland say about how hippocampal learning is different from cortex learning?
The cortex requires a gradient-descent learning rue and multiple passes through the training data. The hippocampus can do it in one shot.
How does the hippocampus train the cortex?
by replaying stored memories so that the gradient descent learning rules can do its job.
What are the types of sells supporting spatial reasoning in the hippocampal system?
Place cells and Grid cells
How do place cells work?
Forming a cognitive map of the physical space
What are some properties of place fields?
They’re present instantly in a new place but take 10-20 minutes to fully develop.
They can be controlled by distal cues. Move a black space on a wall to a new wall and the rat formed a new cognitive map.
They persist in the dark though so they don’t depend on visual cues. Possibly driven by path integration
The map is not topographic
Only 1/3 of place cells have fields in a typical small environment
How did they show that place cells have unrelated fields in different environments?
They put rats in a cylindrical and square arena and measured neural activity. SNR
Also, if the box is drastically bigger, the rat remaps, but if its only slightly bigger, the cells’ fields expand.
How did they show path integration in rodents?
They turn out the lights and have a mom gerbil retrieve her young. She follows the wall on the way there, but then takes a direct route back. They showed that she wasn’t using scent to guid her because if they rotated the box, the mom went to where she would have come from if they hadn’t moved the box.
How do place cells work together with the path integrator?
Place cells learn and maintain the correspondence between local view representation and the path integrator coordinates.
What is the gist of attractor bumps?
Input on the exterior of the receptive fields moves the bump but if its far away, it’s generally ignored. If its constant and repetitive though the rat begins to think its not where it thought it was and updates the map and the bump jumps.
Where in the hippocampus do cells get lost and gained?
Dental gyrus
How can the hippocampus have a different place map for every area if its not topographic?
The attractor network makes this possible. Connections that are not part of the current map look like noise, so the map is formed through specific combinations of connections.
What is Task-Dependent Hippocampal Remapping?
In the cross or circle maze, they recorded cells while the rat was on different parts of a Plus or figure 8 figure. Each map involved different tasks but that amounted to essentially visiting all four corners. Some cells fired regardless of wether it was a plus or figure 8 and some fired only for one of the maps.
What is experience-dependent remapping?
Rats don’t always remap immediately. Onset can be delayed and this shows that remapping isn’t just triggered by a change in input stimulus. Something internal must be happening.
Also, remapping can be partial and can be gradual
What experiment is associated with delayed abrupt complete remapping?
Rats are in a cylinder and a white cue card is swapped for a black one. Most rats do not initially remap based on this change of color, but over time more did remap. Once they created a different map for the two cue cards, they continued to switch maps when the new cue card was shown.
Explain gradual remapping.
Discordant responses. During the remapping process, some cells followed local cues, some distal, and the extent of the remapping increased over days. This may show that the rat is waiting and becoming more and more certain that the environments are reliably different.
Why does remapping matter?
If a rat is trained that it will receive a reward in one location for a white cue card and in another location for a black cue card, it needs to remap in order to “get the task.” Rats who don’t remap never get the task. One rat didn’t remap for 11 days, but on the day it remapped, it got the task.
What is hippocampal replay?
They observed rats standing still and their electrical wave activity as a theta wave and the place cells fire while the rat imagines going in different directions, including backwards or directions the rat has never gone before.
Where is the Path Integrator?
Entorhinal cortex in grid cells. Mosers won the Nobel prize in 2014 for figuring this out.
What evidence is there that human hippocampus is also responsible for spatial memory?
London cab drivers have larger posterior hippocampi than controls.
When remembering complex routes, cab drivers show increased hippocampal activity in right posterior hippocampus but no increase when remember landmarks.
What is the overview of this lecture? What does the hippocampus do?
Build sparse random representations of a complex array of sensory and behavioral information
Learn spatiotemporal associations between these, within appropriate context: learning paths to a goal, learning odor sequences
Retain representations for later use/ consolidation: Replay of paths during sleep, recall of task state after delay: (DMS and DNMS tasks, trace conditioning)
What is DMS and DNMS
Delayed match to sample and delayed non-match to sample. (odor task)
What is trace conditioning?
Conditioning in which an unconditioned stimulus is paired with a conditioned stimulus but a delay happens between them.
